Recently, individual users of computing devices have begun to handle large amounts of information. Consequently, the demand for higher density magnetic recording apparatus, representatively hard disk drives (hereinafter, referred to as HDDs); are intensifying more and more. To attain a higher density HDD, it is important to not only increase the linear recording density in the circular direction but also increase the number of tracks spaced in the radial direction of the disk, namely raise Tpi (Tracks per inch). To realize a higher Tpi, it is essential to shorten the track pitch and narrow the track width. To accurately read data recorded on a narrow track, it is important not to read data from adjacent tracks (ie. not to crosstalk). This makes it necessary to narrow the width of the read device.
Read devices in today's HDDs use GMR (Giant Magneto-Resistance) or TMR (Tunnel Magneto-Resistance). FIG. 3 shows the structure of a typical read device located on a track. The read device includes a pinned layer 21 of which magnetization is fixed in one direction and a free layer 22 of which magnetization can rotate freely subject to the external magnetic field. An intermediate layer 23 between the pinned layer 21 and the free layer 22 is a conductive thin film if the read device is a GMR device or an insulation film in the case of a TMR device. Influenced by a magnetic field 27 from the magnetic recording medium 20, the magnetization of the free layer 22 rotates. This rotation angle (θ) 24 differs depending on the direction of the magnetic field 27 from the medium 20. The electric resistance of the whole device changes depending on the relative angle between the magnetization of the pinned layer and the magnetization of the free layer. Information on the medium is read by using this change.
Either of the GMR and TMR devices include what is known as an in-plane magnetization film which is magnetized along the film surface. To fix the magnetization in one in-plane direction, the shape magnetic anisotropy of the device is utilized. However, if the track width 26 is narrowed, the device width 25 must also be narrowed. This weakens the effect of the shape anisotropy and makes the in-plane magnetization curled, resulting in an instable output. This problem is not unique to read devices in HDDs. In the case of the MRAM (Magnetic Random Access Memory), if the cell aspect ratio (length-to-width ratio) is decreased to allow a higher cell density, the in-plane magnetization of the cell may become so curled as to make it impossible to identify “1” or “0” as the information.
Thus, as described by N. Nishimura et. al. “Magnetic tunnel junction device with perpendicular magnetization films for high-density magnetic random access memory” Journal of Applied Physics, Vol. 91 (No. 8), p. 5246-5249, 2002 (“Non-Patent Document 1”), it is proposed to use a perpendicular magnetization film instead of an in-plane magnetization film. The perpendicular magnetization film is magnetized perpendicularly to the film surface and has a merit that magnetization can be kept perpendicular to the film surface not depending on the aspect ratio of the device or cell. As described in Non-Patent Document 1, and by S. C. Li et. al. “Effects of MgO tunnel barrier thickness on magnetohysteresis in perpendicularly magnetized magnetic tunnel junctions of GdFeCo/FeCo/MgO/FeCo/TbFeCo”, Journal of Applied Physics, Vol. 99, p. 08T310, 2006 (“Non-Patent Document 2”), and by T. Hatori et. al. “MTJ elements with MgO barrier using RE-TM amorphous layers for perpendicular MRAM”, IEEE, Trans. Magn. Vol. 43 (No. 6), p. 2331-2333, 2007 (“Non-Patent Document 3”), a typical data read method uses a magnetic tunnel junction (hereinafter, referred to as MTJ) with perpendicular magnetization films and exploits its tunnel magneto-resistance effect.
Disclosed in JP-A-2000-306376 (“Patent Document 1”) is a fine galvanomagnetic device which utilizes the anomalous Hall effect in a multi-layered film comprising a first ferromagnetic layer with magnetic anisotropy perpendicular to the film surface and a second ferromagnetic layer with magnetic anisotropy perpendicular to the film surface. These layers are magnetically coupled via an insulation layer. The read process does not require re-write operation.
The electric resistance of a perpendicular MTJ structure becomes the lowest when the magnetization of the free layer is parallel to that of the pinned layer disposed on the opposite side of the insulation layer, and becomes the highest when they are in antiparallel. If switching the magnetization arrangement between parallel and antiparallel causes a larger change of the electric resistance, the device sensitivity is considered higher. However, to reverse the perpendicular magnetization of the free layer in such a MTJ structure, a high magnetic field of at least 5 kA/m is required. To use a perpendicular MTJ as a read device, it may be desirable that the magnetic field to switch the magnetization of the free layer be lowered to a level of several hundred A/m-2 kA/m. For application as a memory device, it may be desirable to lower the magnetic field required to write to cells in view of reducing power consumption. Further, the quality of the barrier film in the tunnel junction structure is a critical factor in determining the device performance. Although Al or Mg oxide films are mainly used as barrier films, it is not easy to form these films with several nanometer level uniformity.
In the case of devices (memory devices) using the anomalous Hall effect, as described in Patent Document 1, it may be desirable to lower the magnetic field required to reverse the magnetization of the perpendicular magnetization film.
However, since a perpendicular magnetization film generally exhibits a large demagnetizing field perpendicular to the film surface, it is not easy to attain sufficiently soft magnetic properties, namely several hundred A/m in terms of the reversal magnetic field. To use a perpendicular magnetization film for read elements or memory devices, it may be important in design to lower the magnetization reversal field and facilitate the fabrication.
To use a perpendicular magnetization film in a magnetic sensor or magnetic memory device, it may be important to lower its coercivity. It may also be important to design the device which does not require the preparation of homogeneously thick insulation layer.